This invention relates to systems of the type that are operative to grind material as well as to separate the ground material into a finished product containing particles which are of a predetermined size, and in particular to a high efficiency separator system which embodies a high efficiency mechanical air separator.
For purposes of the discussion that follows hereinafter, systems of the type to which the present invention is directed will be viewed as being composed essentially of two major operating components; namely, a grinding device and a separator device. Regarding first the grinding device, it has long been known in the prior art to provide apparatus which are employable for purposes of effectuating the grinding of materials. To this end, the prior art is replete with examples of various types of apparatus that have been used heretofore to effect the grinding of a multiplicity of different kinds of materials. In this regard, in many instances discernible differences of a structural nature can be found to exist between individual ones of the aforesaid apparatus. The existence of such differences is, in turn, attributable for the most part to the diverse functional requirements that are associated with the specific applications in which such apparatus are designed to be employed. For instance, in the selection of the particular type of apparatus that is to be utilized for a specific application one of the principal factors to which consideration must be given is that of the nature of the material that is to be ground in the application. Another factor to which consideration must be given is that of the fineness to which it is desired to grind the material.
Turning next to a consideration of separator devices, as the name given thereto implies, the function of a separator device is to effectuate in some preestablished fashion a separation of material which is made to enter the separator device. Apparatus have long been known to be available in the prior art which are suitable for use for such a purpose. By way of exemplification in this regard, one such apparatus is that known to those skilled in the art of separator devices as a mechanical air separator. Applicant's assignee is a manufacturer of such mechanical air separators.
With further reference to the mechanical air separators manufactured by applicant's assignee, in accordance with the nature of the construction that such mechanical air separators have heretofore embodied the material which is to undergo separation enters a hollow shaft, or center feed pipe, at the top of the mechanical air separator and under the influence of gravity drops upon a rotating distributor plate which is suitably positioned therebelow. This distributor plate disperses the material into the upward sweep of circulating air which is suitably developed by a fan located in the top chamber of the mechanical air separator. One or more whizzer blades are operative to bring about a centrifugal motion of the air and the material. The effect thereof in turn is to concentrate the oversize material along the inner cone of the mechanical air separator whereupon the oversized material passes out of a tailings spout which is located at the bottom of the mechanical air separator. Meanwhile, the air and powdered material of the desired fineness moves through the fan and is delivered into the outer cone chamber of the mechanical air separator whereupon the fines are discharged from the mechanical air separator as finished product. The air, after the fines have been released in the outer cone, returns through the deflector ports with which the mechanical air separator is provided to the inner cone thereby establishing a continuous circulation of the flow of air.
Mechanical air separators of the type that have heretofore been manufactured by applicant's assignee and which operate in the manner that has been described hereinabove have been found to be suitable for use for many different purposes. By way of exemplification and not limitation, such mechanical air separators have been successfully employed in closed circuit grinding operations for classifying, and drying where desired, raw mix, and classifying, and cooling, finished cement; for producing limestone sand to meet close specifications of granular sand material to be used in bituminous concrete, mortar, as an aggregate and many other uses; for making fine, uniform cake mixes and for the production of protein-enriched grades of flour; for producing a high fineness, uniformly classified, hydrated lime for chemical and spray purposes; for classifying numerous food products including sugar, cocoa, milk powder, food mixtures with various ingredients, corn starch and wheat starch, and soya bean meal; in applications where manufactured chemicals are required in closely sized form, i.e., for making the various grades ranging from extremely fine to the granular dustfree gradations of such chemicals as soda ash and sodium phosphate; in the beneficiation of certain materials such as talc, kaolin and clays, and phosphate rock for purposes of removing therefrom impurities in the form of silica, flint and other foreign materials; for classifying metal powders consisting of copper, bronze, iron and various alloys and for de-dusting of seacoal for foundry facing use, etc.
For purposes of its use in applications of the sort enumerated above, it is possible to combine the aforereferenced mechanical air separator with a variety of different forms of grinding devices such as ball mills, tube mills, compartment mills, etc. Furthermore, when so employed in combination with a grinding device, the mechanical air separator may be connected either in closed circuit relation or in open circuit relation therewith. When operated in closed circuit combination with a grinding device, the mechanical air separator is designed to skim off the fines as fast as they are produced such that the grinding device works only on fresh material without wasting power. The tailings from the mechanical air separator, however, are discharged back to the grinding device for further reduction. After being reground in the grinding device, the reground material is returned to the mechanical air separator along with the feed that is being supplied thereto so that a constant circulating load is established between the grinding device and the mechanical air separator. In contradistinction to the manner in which the mechanical air separator is made to interact with the grinding device when the former is connected in closed circuit relation therewith, when the mechanical air separator is connected in open circuit relation with the grinding device the ground material is supplied from the grinding device in the form of feed to the mechanical air separator but the tailings from the mechanical air separator are not recirculated to the grinding device for further reduction in the grinding device.
One application in which particular use has been made of mechanical air separators is that relating to the grinding and classifying of cement wherein the mechanical air separator has been combined with a grinding device so as to form a closed circuit therewith. However, notwithstanding the extent to which mechanical air separators have been employed heretofore for this purpose the efficiency of mechanical air separators has for many years nevertheless been a subject of considerable concern. Moreover, much of this concern has come from the cement industry itself with respect to the finish cement grinding circuit. In the past, though, measurement of the grinding circuit's circulating load, recovery and efficiency was usually based on a single particle size measurement of the feed, fines and tailings. Also, the methods normally employed for this purpose have been subject to large errors. Though some analysis has been done utilizing sub-sieve particle size equipment, it has been limited by the particle size range and time requirements of the analyzers. This had made extensive study of the performance of mechanical air separators impractical.
Very recently, however, particle size analyzers have become available that can analyze samples very quickly. They also provide a complete particle size distribution for the sample. This in turn has permitted an extensive study of the performance of mechanical air separators to be undertaken at a reasonable cost. In addition, it has permitted results to be based on the complete particle size distribution and has enabled errors to be recognized.
As regards the matter of efficiency, the higher the efficiency of a mechanical air separator, the closer the fractional recovery comes to 100% at the finest particle sizes. The term fractional recovery refers to the percentage of the material of a given particle size or between two particle sizes which is present in the feed to the mechanical air separator and which is recovered in the finished product from the mechanical air separator. When fractional recovery is plotted against particle size, this plot is referred to as a "Tromp Curve". A "Tromp Curve" is one way of measuring the efficiency of a mechanical air separator. In this regard, a perfect mechanical air separator, i.e., one having an efficiency of 100%, would have a "Tromp Curve" that would be a vertical line at the "cut point" particle size. That is, everything in the finished product would be finer than this cut point particle size and everything in the tailings would be coarser.
A mechanical air separator's inability to attain 100% fractional recovery is referred to as bypassing. More specifically, bypassing is defined as being the difference between a numerical value of 100% and the amount of fractional recovery that is actually attained at the finest particle sizes. Based on test results, it has been determined that mechanical air separators with low circulating loads have the least bypass, i.e., are the most efficient, whereas mechanical air separators with high circulating loads have the greatest bypass, i.e., are the least efficient.
Bypassing is alleged to be caused by one or more of the following three events. One of these is the internal recirculation of fines. A second is the inadequate dispersion of the feed in the air prior to the feed reaching the classifying zone of the mechanical air separator. The third is the existence of an excessive material/air ratio which has the effect of causing interference between particles within the classifying zone of the mechanical air separator.
For a number of years, the cyclone separator has been marketed in an attempt to alleviate the first of the three causes of bypassing that has been enumerated above. Unfortunately, however, the cyclone separators that have been marketed utilize relatively low efficiency cyclones, i.e., cyclones having an efficiency of between 88 and 92%, and thus the effect thereof is that fines are still recirculated to the separator. In addition, such cyclone separators have not addressed the second cause of bypassing that has been enumerated above. As such, the rather small improvement in efficiency achieved with the cyclone type separator generally has not justified the high capital cost of such cyclone separators. It is known that in some instances repeated washings of the tailings has been undertaken in the cyclone separator in an attempt to remove the fines from the tailings. The cyclone separators are so large, though, that the low velocity air moving through them cannot lift the fine material back up to the classifying zone of the cyclone separator.
Thus, there has been evidenced in the prior art a need for a new and improved form of mechanical air separator which will embody a mode of operation whereby the causes of bypassing enumerated hereinabove would either be entirely obviated or at a minimum significantly reduced such that the efficiency of the mechanical air separator would be measurably improved. Namely, a need has been evidenced for a new and improved form of mechanical air separator that would enable one to realize through the use thereof economies in power consumption in the grinding circuit and steeper particle size distribution in the finished product. That is, a new and improved form of mechanical air separator has been sought which would be operative to reduce the specific horsepower requirements of the grinding circuit thereby enabling the circuit to be operated at higher capacity as well as permitting increased flow of air through the device and even in some instances the elimination of the need for water sprays and cement coolers to effect cooling. In addition, there has been sought such a mechanical air separator which is further characterized in that the effectiveness of particle size separation achievable therewith is such that it is possible to realize much higher fineness at the present Blaine and thereby higher cement strengths than would normally result at the same Blaine.
It is, therefore, an object of the present invention to provide a new and improved high efficiency separator system that is operative both to grind material and to thereafter separate the ground material into a finished product which contains particles that are of a predetermined size.
It is another object of the present invention to provide such a new and improved high efficiency separator system that includes a grinding device in which the material is ground and a new and improved high efficiency mechanical air separator in which the classification of the ground material into a finished product is accomplished.
It is still another object of the present invention to provide such a new and improved high efficiency separator in which all of the air vented from the grinding device is fed directly to the high efficiency mechanical air separator thereby effectuating the removal from the grinding device of material that has been ground to an acceptable fineness which if not otherwise removed would continue to undergo grinding needlessly in the grinding device.
A further object of the present invention is to provide such a new and improved high efficiency separator system wherein a characteristic of the high efficiency mechanical air separator that is employed therein is that air is passed through the high efficiency mechanical air separator only once thereby preventing the internal recirculation of fines therethrough, which has been identified to be a cause of bypass.
A still further object of the present invention is to provide such a new and improved high efficiency separator system wherein a characteristic of the new and improved high efficiency mechancial air separator that is employed therein is that the flow of air therethrough is controlled so that a high velocity mixing zone is established therein whereby excellent mixing of the air and feed material is achieved prior to the feed material reaching the classifying zone within the high efficiency mechanical air separator thus ensuring the avoidance of inadequate dispersion of the feed material in the air, which has been identified to be a cause of bypass.
Yet another object of the present invention is to provide such a new and improved high efficiency separator system wherein a characteristic of the new and improved high efficiency mechanical air separator that is employed therein is that a maximum limit is established for the material/air ratio employed therewith in an effort to prevent the occurrence of interference between particles in the classifying zone of the high efficiency mechanical air separator, which has been identified to be a cause of bypass.
Yet still another object of the present invention is to provide such a new and improved high efficiency separator system which is characterized in that through the use thereof economies in power consumption in the grinding circuit are capable of being realized and a steeper particle size distribution in the finished product is capable of being achieved.
Yet still a further object of the present invention is to provide such a new and improved high efficiency separator system wherein a characteristic of the new and improved high efficiency mechanical air separator that is employed therein is that the latter is suitable for use both in new applications and in retrofit applications.